Piezoelectric apparatus and method



Feb. 1, 1944. R. s. BAILEY PIEZO-ELECTRIG APPARATUS AND METHOD Filed Aug. 20, 1941 FIGURE Q m R F O F I H I T Mu Ap .Ww

FIGURE 2 FREQUENCY F/GUREB DE TE C TOR TUNED AMPLIFIER F/GURE'4 Richard S. Bailey INVENTOR.

BY (9 Au ATTORNEY.

I Patented Feb. 1, 1944 UlTED STATES PATENT orric PEEZOELEG'I BIC APPARATUS AND METHOD Richard S. Bailey. Baltimore, Md.. assignor to The common feature During, and before facturing processes Bendix Aviation Corporation, South Bend, Ind, a corporation of Delaware Application August 20, ic n, Serial No. 407,615

, is Claims.

This invention relates to the manufacture of the determination of the resonant frequency of a piezo-electric body during machining operations thereon.

lhephenomenon of piezo-electricity is well known in the art. Crystals exhibiting the piezopiezo-electric bodies, and, more particularly, to.

electric efiect will produce, on opposite faces, op-

'posite electric charges when the crystal is deformed by the application oi force. Conversely, application of an electric field to the crystal will result in deformation of the crystal. If an external force be applied to the crystal, and this the frequency of mechanical vibration, and

hence is directly indicative of the natural resonance frequency of the crystal. It is this latter efiect, which has hitherto found little or no application, that is Piezo-electric quartz crystals are widely employed to control the frequency of oscillators wherein great stability and precision of the oututilized in the apparatus and method of my invention.

put frequency is required. Several forms of oscillators of this type are described in Chapter 5 of The Radio Amateurs Handbook for 1941 as published by The American Radio Relay League. of these oscillators, is an arrangement whereby electrical energy is fed to the crystal in a. manner causing it continually to vibrate at its own natural frequency, and this freuency, in turn, determines the output frequency of the system.

In practice, what has been referred to as a crystal is, in reality, a small plate cut out of the mother crystal, the dimensions of which are determined by the desired operating frequency.

the completion of the manuthis plate is called a blank. After completlonof blank is termed a crystal. In the manufacture of medium and high frequency crystals, the length and width of the blank are normally predetermined, and the natural resonance frequency of the blank is adjusted to theselected value by varying its thickness. A typical procedure for the prior art manufacture of piezo-electric quartz crystals is as follows.

A bar whose cross-sectional dimensions are equal to the desired length and width of the finished crystal is cut out of the mother crystal and a series of plates, or blanks, whose thickness is substantially greater than-that giving the desired natural, period is sawed out of the bar. At this stage in the operation the thickness of the blanks usually varies considerably due to the inaccuracy of the sawing operation so, a number of these blanks are then placed in a lapping machine where they are simultaneously ground by abrasion to a uniform thickness, and the thickness is then further reduced in this lapping machine until the natural period of the blanks is very slightly greater than the desired final value. During this procedure, the machine is frequently stopped and frequency measurements are made on selected blanks. The blanks are now removed from the machine and finally adjusted one at a time to the desired frequency by a hand lapping operation, the frequency being closely checked by further measurements during the hand lapping.

The control of the machine lapping operation is obviously of great importance, for if the blanks become too thin they must be discarded, and if they are removed while still too thick an excessive amount of the more expensive hand lapping is required. In eithercase, the cost of production is greatly increased. Two general methods, or combinations of these methods, are now used in determining when to stop the machine lapping process. In the first of them, as above stated, the

lapping machine is frequently stopped, a sample blank removed, and its thickness or resonance frequency determined. It is obvious that this method materially reduces the output from the lapping machine. Further, should there be misall operations, the finished alignment between the lapping surfaces, control of the thickness of a single sample blank does not "insure that some otherblank will not be lapped too thin. In the second method, the approximate lapping time is determined by use of the first method initially, and then, on subsequent lots, the total lapping time is controlled to the same value. Inaccuracies are introduced-in this method by variations in speed of the lapping surfaces, and

by variations in the quantity and quality of the abrasive employed. Usually, when approaching the end of the lapping period, the timing method is supplemented by the first described method.

In the invention to be disclosed herein, the difli culties and shortcomings above outlined are obviated, and finished crystals may b produced in larger volume at lower unit cost, therefore:

One of the principal objects of my invention is to provide method and apparatus for more economically producing piezo-electric crystals, by increasing machine output duringa given period of time.

Another object of my invention i to provide method and apparatus for continuously indicating the natural resonance frequency of a piezoelectric body during a machining operation on said body, without interrupting said operation.

A further object of my invention is to provide method and apparatus obviating the close attention previously required in the processing of piezoelectric crystals.

Yet another object of my invention is to provide method and apparatus for automatically controlling a machining operation on a piezoelectric body.

A still further object of my invention is to provide method and apparatus for ascertaining a critical dimension of a piezo-electric body during a machining operation on said body.

Still another object of my invention is to provide apparatus for the interruption of a lapping operation on a piezo-electric body when the natural resonance frequency of said body reaches a predetermined value.

Other objects and advantages will in part be disclosed and in part be obvious when the following specification is read in'conjunction with the drawing, in which:

Figure 1 is a diagrammatic epresentation of a preferred form of my invention adapted to measure the natural resonance frequency of piezo-electric crystals in a lapping machine.

Figure 2 is a graph showing a suitable transmission characteristic for the amplifier used in Figure 1. V operational characteristic for the frequency meter used in Figure 1. P

Figure 3 is a diagrammatic representation of an alternative form of my invention adapted to interrupt automatically the lapping process when the natural resonance frequency of the piezo-electric crystals reach a predetermined value.

- Figure 4 is a graphical representation of a suitable operating characteristic for the combined amplifier and detector used in Figure 3. Figure 4, in addition, indicates the drop out current for the relay of Figure 3, to assist in the visualization of the operation of the apparatus.

In the drawing, like parts are like reference characters.

Referring now in greater detail to Figure designated by ,number of piezo-electric crystal blanks 5 are mounted in the work-table I of a lapping machine which is provided with shallow locating.

recesses adapted to receive and hold the blanks. The lap 2 is charged with aprepared abrasive material -and rotated over the top surface .of the blanks through the action of the motor l as transmitted by'the shaft 3. Thus, the material of the blanks is gradually removed by abrasive action, and in this manner they may be reduced to any desired thickness. Operation of the machine is controlled by the line-switch 6. 'The abov combination is well known in the art and constitutes the elements of a. widely used form of lapping machine.

The motor housing is grounded-at 1, thereby effectively grounding the lap 2 through the 'shaft be employed, and the ungrounded input terminal 7 of the amplifier l0 may be efiectively connected to the work-table I, by direct, capacitive, Or in-' ductive coupling, while the grounded input terminal of the amplifier Ill may be effectively connected to the lap 2 through the ground ll, ground I, motor housing 4 and shaft 3. A frequency meter I l may be connected to the output terminals I2, I 3 of amplifier It). This frequency meter may be of the type shown, utilizing a series resonant circuit comprising inductor l5 .and capacitor II, the current through the circuit actuating radio frequency milliammeter it which may be calibrated in terms of the impressed frequency. The amplifier l0 may preferably incorporate an automatic gain control, maintaining Also shown in Figure 2 is a suitable fied and impressed 3- and the bearings (not shown) which are inciuded as a necessary part of the lapping machine. The work-table; rests on the base 8 and may be insulatedfrom ground by the presence I the latching .relay 2|.

the voltage applied to the frequency meter l4 essentially constant for any value of input voltage applied to the input terminals of the amplifier. e In Figure 2, curve I! represents a suitable transmission gain vs. frequency characteristic for the amplifier Ill. The frequency to which the crystals are being adjusted may lie between the central and upper portions of this characteristic. Curve l8 graphically portrays a suitable operating characteristic for frequency meter It, the abscissa indicating the frequency, while the ordinate indicates the deflection of milliammeter l6 .at that frequency, the impressed voltage being constant. The resonant circuit including inductor l5 and capacitor l1 maybe preferably so ad- Justed that the desired crystal response frequency f0 lies on the low frequency slope of the resonance curve l8.

In the practice of my invention, the machine lapping of the blanks may be performed in the usual manner. However, the necessity for periodic interruption of this process is now eliminated. The intermittent mechanical forces exerted on the crystal blanks during the lapping operation cause the blanks to vibrate at their i:

' tacting surfaces, whence "it is coveyed to the in- Here the currents are amplion the frequency meter l4, whereupon the output frequency is indicated by the properly calibrated milliammeter it. Thus, by observation of the indications of the meter l6, and without any interruption of the lapping process, the operator has continuously available during the operation of the machine precise knowledge of the natural resonance frequency of the crystal blanks which are being lapped.

Referring now to Figure 3, I have shown a form of my invention adapted to automatically interrupt the lapping process when the response frequency of the piezo-electric blanks reaches a predetermined value. The lapping machine is constructed and connected to the input terminals of an amplifier in the same manner as described above. A tuned amplifier I! may be employed, which may combine the functions of amplification and frequency determination. The output of amplifier l9 may be connected to a detector 20 which may be of the grid leak type, and in the plate circuit of this detector may be connected Thecontacts 22 -of the latching relay 2| may be connected to interrupt the powersupply connections to the motor I.

put of amplifier l0.

In the relay shown, contacts 22 are normally held closed against the restoring force of the spring 23 by the magnetic effect of the plate currentof the detector 20. When the plate current ofthe detector falls below a critical value, the contacts are opened by the spring 23and are latched in this position by the latch bar 24 which is pivoted.

lationship between the detector plate current and the input frequency to amplifierlS, input voltage being constant. The desired crystal frequency may preferably lie on the low frequency slope of this resonance curve. The dashed line 25 of the same figure represents the value of current at which the latching relay contacts open.

In the practice of this form of my invention, the lapping process is begun in the usual manner, and amplifier i9 is so adjusted that crystal response frequency f0 falls on the desired'portion of the resonance curve 25. The remainder of the operation is automatic, thus freeing the operator for other duties. When thefrequency of the energy applied to the input of amplifier 89 reaches the desired point on the resonance curve, the plate current of detector 28 decreases to a value less than that required to maintain relay 2! in the operated position, thereby permitting contacts 22 to open and interrupt the power supply connections to the motor 3 thus terminatingthe' lapping operation.

in view of the fact that the natural frequency of a crystal is dependent on its dimensions, it is obvious that my invention may be employed to determine any dimension affecting the natural resonance frequency of a piezo-electric crystal.

'2 have found that the method and apparatus of my invention may be readily employed in conjunction with harmonics of the natural resonance frequency of a piezo-electric crystal, and it is my intention that this mode of operation fall within the scope of the appended claims.

While I have'described the application of my invention to the lapping of piezo-electric quartz crystals, it is neither my intention nor desire to limit the scope of my invention to this particular process or this particular material. The term machining operation in the appended claims is intended to describe any mechanical operation for the removal of material from a body, which produces the necessary internalstresses for the generation of alternating piezo-electric currents in that body, such as filing-milling or turning.

Where reference is made to a piezo-electric substance, it is intended to describe not only piezoelectric quartz, but any substance exhibiting the 'piezo-electric effect, such as tourmaline or R0- chelle salts.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claims:

What I claim is:

l.' The method of determining the natural res- 'onance frequency of a piezo-electric body undergoing a machining operation, which comstresses caused by said operation.

.quencl T. which comprises the steps of moving a' piezo-electric crystal blank in contact with and relative to a reducing .means, amplifying the plezo-electric current generated by said motion, measuring the frequency'of said piezo-electric current during the continuation of said motion, and interrupting the reducing operation when the frequency of said piezo-electric current becomes substantially equal to said predetermined frequency. 7 V

3. The process of manufacturing a piezo-electric crystal having a predetermined response frequency which comprises mechanically removing material from a blank of piwo-electric materiaLdetermining the response frequency of said blank by measuring the frequency of the piezo-electric currents generated by and during the continuation of said removal of material and I interrupting the removal of material when said frequency of said current reaches a predeter- V mined value.

2. The method of'manufacturing a piezo-electric crystal having a predetermined response-fre- 4. In the process of machining a piezo-electrio body, the method of determining a critical dimension of said body, which comprises measuring the frequency of the piezo-electric currents generated in said body by the internal stresses produced during said operation.

5. In apparatus for the manufacture of a piezoelectric body, the combination of a work-holder adapted to receive said body, a tool movable with respect to said work-holder, an amplifier responsive to the piezo-electric currents produced in said body by the worklng'operation and having input terminals connected efiectively to the work-holder and to the tool, and frequency indicating'means connected to the output .of said amplifier.

6. In apparatus for the manufacture of a piezoelectric body, the combination of a work-holder adapted to receive said body, a tool movable with respect to said work-holder, an. amplifier tuned to a'predetermined frequency and having inputterminals effectively connected to said work-hold-.

er and to said tool, detecting means connected to the output of said amplifier, and current amplitude responsive indicating means connected to the output of said detector. I

7. In apparatus for the manufacture of a piezoelectric body, the combination of pa work-holder adapted to receive said body, a tool movable with respect to said work-holder, an amplifier tuned to a predetermined frequency and having input terminals efiectively connected to said work-holder and to said tool, and amplitude responsive indicating means connected to the output of said amplifier.

8. In apparatus for the manufacture of a piezoelectric body, the combination of a work-holder adapted to receive said body, a tool-movable with respect to said work-holder, a detector tuned to a predetermined frequency and having input terminals effectively connected to said work-holder and to said tool, and current amplitude responsive indicating means connected to the output of said detector.

9. In apparatus for the manufacture of a piezoelectric body, the combination of a work-holder adapted to receive said body, a tool movable with respect to said work-holder, and means connected to said work-holder and to said tool to determine the frequency of the alternating voltage generated within said piezo-electric body by the internal stresses produced therein as a result of and during the continuation of the working operatfon on said body and appearing at said workholder andmaid tool.

10. In apparatus for the manufacture of a piezo-electricbody, the-combination of a work- 'holder adapted to receive said body, a tool mov- 1 frequency responsive means to said work-holder and to said tool, said connecting means and said 20 frequency responsive means presenting a positive input conductance when viewed from said body, and means actuated by said frequency responsive means for controlling the machining operation.

12. In apparatus for the manufacture of piezoelectric crystals, the combination of a lapping machine adapted to receive a piezo-electric crystal blank, and control means arranged to interrupt the lapping process .when the frequency of the piezo-electric currents produced in said blank by the internal stresses resulting from and during the continuation of the operation of said lapping machine reaches a predetermined value. 13. The method of manufacturing a piezo-electric crystal having a predetermined response frequency, which comprises the steps of reducing a dimension of a piezo-electric crystal blank by mechanical means, amplifying the piezo-eiectric current generated by said blank during and as a result of said mechanical reduction, measuring the frequency of said piezo-electric current during the continuationof the operation of said mechanical means on said blank. and interrupting the reducing operation when the frequency of said piezo-electric current becomes substantially equal to said predetermined frequency.

RICHARD S. BAILEY. 

